1. Field of the Invention
The present invention relates generally to the field of integrated circuit fabrication, and more specifically to the detection of residual photoresist through fluorescence.
2. Description of the Related Art
Photolithography is commonly used for fabricating integrated circuits. Photolithography involves coating a surface to be etched, such as a semiconductor wafer, with photoresist and exposing the photoresist to light (usually at a specific wavelength) according to a desired pattern. Depending upon whether negative or positive photoresist is used, either the portions of the photoresist exposed to the light or the portions of the photoresist not exposed to the light are removed, leaving behind a photoresist mask. This process is referred to as developing the photoresist. The photoresist mask protects the portions of the surface covered by the mask while the portions of the surface not covered by the mask are etched, thereby producing a desired pattern (the negative of the mask) in the surface.
The photoresist mask must be removed after the surface has been etched, as any photoresist not removed will adversely affect subsequent processing of the wafer and/or performance of the completed integrated circuit. However, removing photoresist is not always easy. Typically, the photoresist is dry-etched and then a cleaning solution, such as a piranha solution (used when non-metals have been etched) or hot phosphoric acid (used when metals are etched), is used to wash away photoresist. Often photoresist is left behind or, after it has initially been dislodged by the cleaning solution, reattaches to another area of the surface. Photoresist not removed by the cleaning process is referred to herein as residual photoresist. Because residual photoresist adversely affects the final product, it is important to detect residual photoresist so it can be removed.
One method to detect residual photoresist is to visually inspect the surface with the aid of an optical microscope. This method is not very effective, however. It is difficult to distinguish photoresist from many surfaces, and such techniques are inherently time-consuming, must be done manually, and are therefore expensive.
Another method for detecting residual photoresist takes advantage of fluorescence. Fluorescence is the emission of light or other electromagnetic radiation of a longer wavelength by a substance as a result of the absorption of shorter wavelengths, provided the emission continues only as long as the stimulus producing it is maintained. Organic materials such as photoresist have characteristic light absorption and emission patterns, or spectra. The wavelength of radiation that causes fluorescence shall be referred to herein as the excitation wavelength, while the wavelength of the emitted radiation shall be referred to herein as the emission wavelength. It should be understood, however, that both xe2x80x9cexcitation wavelengthxe2x80x9d and xe2x80x9cemission wavelengthxe2x80x9d refer to a range of wavelengths norminally represented by the single wavelength referred to. In other words, if a substance is said to have an excitation wavelength of 325 nm, it should be understood that radiation at a wavelength of approximately 325 nm will cause the substance to emit radiation to varying degreesxe2x88x92325 nm represents a spectrum of wavelengths approximately equal to 325 nm. The emission wavelength is always shifted toward longer wavelengths (lower energy) as compared to the excitation wavelength. This is known as Stokes"" shift.
Fluorescence can be used to detect residual photoresist by irradiating the surface with radiation at the excitation wavelength of the photoresist and substantially simultaneously detecting light emitted by the photoresist at the emission wavelength. Because the emission wavelength of photoresist is different from the wavelength of the radiation reflected by the surface, any fluorescing residual photoresist is easily detected. Machines such as the apparatus described in U.S. Pat. No. 4,800,282, the contents of which are hereby incorporated by reference herein, have been developed to automatically detect the presence of radiation at the emission wavelength of photoresist, thereby automating the residual photoresist inspection process. Such a machine is often referred to as an ARI (Automatic Resist Inspection) tool.
A problem with the use of ARIs has recently surfaced. Many ARIs in use today work well for photoresists with relatively longer wavelengths. For example, a common ARI in use today is capable of detecting emission wavelengths of approximately 590 nm and above. This ARI is well-suited for use with i-line photoresist, which has an exposure wavelength of approximately 365 nm, an excitation wavelength of approximately 530 nm and an emission wavelength of approximately 600 nm. As integrated circuit geometry continues to shrink, manufacturers have necessarily turned to photoresists with shorter exposure wavelengths to realize the higher resolution required for smaller integrated circuit geometries.
The use of deep ultraviolet (DUV) photoresist, with an exposure wavelength of 248 nm, has become increasingly common (i-line photoresist is opaque at this wavelength). However, the chemistry of DUV photoresist is different from previous photoresists such i-line photoresist. I-line photoresists are typically based on a diazonapthoquinone/novolac resin. DUV photoresists, on the other hand, typically comprise a photopolymer that is a derivative of poly-hydroxy-styrene and an onium salt photoacid generator. The photopolymer should fluoresce at an excitation wavelength of approximately 325 nm and an emission wavelength of approximately 420 nm. However, experience has shown that it does not. It is thought that the onium salt photoacid generator inhibits fluorescence of the DUV photoresist. In any event, the failure of the DUV photoresist to fluoresce; or, even if it did fluoresce, the existence of an emission wavelength shorter than that which can be detected by many existing ARIs (which often have filters that will only pass radiation above 525 nm onto a sample and only pass radiation above 590 nm onto the sensor), presents a serious problem to integrated circuit manufacturers.
What is needed is a photoresist with a shorter exposure wavelength whose residue fluoresces such that existing automatic resist inspection tools can detect its presence.
The invention overcomes to a great extent the aforementioned problems by providing for the addition of materials to enhance the fluorescence of a photoresist such that otherwise undetectable residual photoresist can be detected using existing automatic resist inspection tools. In one embodiment of the invention, a benign tag that does not interfere with the photochemistry of the photoresist is added to the photoresist before it is processed. In a second embodiment of the invention, a tag is introduced onto a surface on which residual photoresist may be present such that the tag is absorbed or adsorbed by the residual photoresist, thereby rendering the residual photoresist easily detectable. The tag may be incorporated into the cleaning solution, or the cleansed wafer may be rinsed with a separate solution containing the tag for subsequent inspection.